Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use as a vehicular power plant to replace the internal combustion engine. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode side of the fuel cell and air or oxygen is supplied as the oxidant to the cathode side. PEM fuel cells include a "membrane electrode assembly" (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the fuel for the vehicle owing to the ease of on-board storage of liquid fuels and the existence of a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a fuel processor, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam and methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to this reaction: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2.
The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur. The heat required to produce enough hydrogen varies with the demand put on the fuel cell system at any given point in time. Accordingly, the heating means for the fuel processor must be capable of operating over a wide range of heat outputs. Heating the fuel processor with heat generated externally from either a flame combustor or a catalytic combustor is known. U.S. patent applications Ser. Nos. 08/975,422 and 08/980,087 filed in the name of William Pettit in November, 1997, and assigned to the assignee of the present invention, disclose an improved catalytic combustor, and the integration thereof with a fuel cell system which fuels the combustor with unreformed liquid fuel, hydrogen-containing anode exhaust gas from the fuel cell, or both. The operating cycle depends on many factors, such as anode stoichiometry, steam/carbon ratio, electrical demand placed on the system, etc.
Load changes placed on the fuel cell resulting in greater or lower power output requirements, results in the fuel processor generating more or less hydrogen. Correspondingly, since the combustor generates whatever heat input is required to sustain the chemical reactions within the fuel processor, the combustor likewise must generate more or less heat to maintain the required reaction temperatures within the fuel processor. The temperature control of the combustor is dependent upon several parameters, an important one being the air flow to the combustor.
What is needed in a vehicular fuel cell application is a fast response to fuel cell load changes. However, air flow control devices using simple feedback to control the air flow to the combustor demonstrate slow response times.
Another problem results from the use of cathode effluent as an air source to the combustor. Such cathode effluent is typically oxygen depleted after exiting the fuel cell such that the actual constituent makeup of the cathode effluent, in terms of water, nitrogen and oxygen differs from that found in normal air. As the air to the combustor is taken from two different sources depending upon the mode of operation of the fuel cell apparatus, i.e., start-up, warm-up, normal operating run mode, etc., conventional sensors which merely measure air or mass flow rates do not take into account the constituent makeup of such air which may have a deleterious effect on the temperature in the combustor.
Thus, it would be desirable to provide an air flow control method for a fuel cell apparatus which has a fast response to load changes, utilizes closed loop control with conventional automotive sensors and actuators and automatically compensates for molar fraction deviations of oxygen depleted air in the air flow stream to the combustor.